1. Technical Field
The present inventions relate to methods and apparatus for making fracturing fluid containing coated particulate materials, such as proppants.
2. Background Art
Hydraulic fracturing is a common stimulation treatment. The purpose of a fracturing treatment is to provide an improved flow path for oil or gas to flow from the hydrocarbon-bearing formation to the wellbore. A treatment fluid adapted for this purpose is sometimes referred to as a “fracturing fluid.” The fracturing fluid is pumped at a sufficiently high flow rate and pressure into the wellbore and into the subterranean formation to create or enhance one or more fractures in the subterranean formation. Creating a fracture means making a new fracture in the formation. Enhancing a fracture means enlarging a pre-existing fracture in the formation.
A newly-created or newly-extended fracture will tend to close together after the pumping of the fracturing fluid is stopped. To prevent the fracture from closing, a material is usually placed in the fracture to keep the fracture propped open and to provide higher fluid conductivity than the matrix of the formation. A material used for this purpose is referred to as a “proppant.”
A proppant is in the form of a solid particulate, which can be suspended in the fracturing fluid, carried downhole and deposited in the fracture to form a proppant pack. The proppant pack props the fracture in an open condition, while allowing fluid flow through the permeability of the pack. The proppant pack in the fracture provides a higher-permeability flow path for the oil or gas to reach the wellbore compared to the permeability of the matrix of the surrounding subterranean formation. This higher-permeability flow path increases oil and gas production from the subterranean formation.
A particulate for use as a proppant is usually selected based on the characteristics of size range, crush strength, and solid stability in the types of fluids that are encountered or used in wells. Preferably, a proppant should not melt, dissolve, or otherwise degrade from the solid state under the downhole conditions.
The proppant is selected to be an appropriate size to prop open the fracture and bridge the fracture width expected to be created by the fracturing conditions and the fracturing fluid. If the proppant is too large, it will not easily pass into a fracture and will screen out too early. If the proppant is too small, it will not provide the fluid conductivity to enhance production. See, for example, McGuire and Sikora, 1960. In the case of fracturing relatively permeable or even tight-gas reservoirs, a proppant pack should provide higher permeability than the matrix of the formation. In the case of fracturing, ultra-low permeable formations, such as shale formations, a proppant pack should provide for higher permeability than the naturally occurring fractures or other microfractures of the fracture complexity.
One common problem is that the proppant may not be sufficiently strong by itself to prop open a fracture. Crushing of proppant by fracture closure would generate fine particulates that migrate in the propped fracture, plug up pore spaces in the proppant pack, choking the flow path and decreasing well production. Proppant flowing back along with the production fluid often result in plugging or eroding of downhole equipment, such as electrical submersible pumps, or erosion of choke valves and other surface equipment. Another common problem is that the surface of the proppant may have an undesirable wettability characteristic for producing oil or gas from a particular subterranean formation. Another common problem is that, as the oil or gas moves through the subterranean formation, it can dislodge and carry particulate with the fluid into the wellbore. The migration of this particulate can plug pores in the formation or proppant pack, decreasing production, in addition to causing abrasive damage to wellbore pumps, tubing, and other equipment.
To help alleviate some of the common problems mentioned above, a resinous material can be coated on the proppant before the proppant is inserted in the formation. The term “coated” does not imply any particular degree of coverage on the proppant particulates, and the coverage can be partial or complete.
For example, some or all of the proppant can be coated with a curable resin. The curable resin can be allowed to cure on the proppant prior to the proppant being introduced into the well. The cured resin coating on the proppant provides a protective shell encapsulating the proppant and keeping the fine particulates in place, if the proppant were crushed or provides a different wettable surface than the proppant without the coating.
A curable resin coating on the proppant can be allowed to cure after the proppant is placed in the subterranean formation for the purpose of consolidating the proppant of a proppant pack to form a “proppant matrix.” As used herein, “proppant matrix” means a closely associated group of proppant particles as a coherent mass of proppant. Typically, a cured resin consolidates the proppant pack into a hardened, permeable, coherent mass. After curing, the resin reinforces the strength of the proppant pack and reduces the flow back of proppant from the proppant pack relative to a similar proppant pack without such a cured resin coating.
A resin or curable resin can be selected from natural resins, synthetic resins, and any combination thereof in any proportion. Natural resins include, but are not limited to, shellac. Synthetic resins include, but are not limited to, epoxies, furans, phenolics, and furfuryl alcohols, and any combination thereof in any proportion. Examples of resins suitable for coating particulates are described in U.S. Pat. Nos. 6,668,926; 6,729,404; and 6,962,200. An example of a suitable commercially available resin is the EXPEDITE™ compound product sold by Halliburton Energy Services, Inc. of Duncan, Oklahoma.
By way of another example, some or all of the proppant can be coated with a tackifying agent, instead of, or in addition to, a curable resin. The tackifying agent acts to consolidate and helps hold together the proppant of a proppant pack to form a proppant matrix. Such a proppant matrix can be flexible rather than hard. The tackifying agent-coated proppant in the subterranean formation tends to cause small particulates, such as fines, to stick to the outside of the proppant. This helps prevent the fines from flowing with a fluid, which could potentially clog the openings to pores.
Tackifying agents include, but are not limited to: polyamides, polyesters, polyethers and polycarbamates, polycarbonates, and any combination thereof in any proportion. Examples of tackifying agents suitable for coating particulates are described in U.S. Pat. Nos. 5,853,048; 5,833,000; 5,582,249; 5,775,425; 5,787,986; and 7,131,491, the relevant disclosures of which are herein incorporated by reference. An example of a suitable commercially available tackifying agent is the SANDWEDGE™ product sold by Halliburton Energy Services, Inc. of Duncan, Oklahoma.
It is advantageous and common to coat proppant particles. The coated proppant particles are suspended in the fracturing fluid, preferably on the fly, and the resulting coated proppant particles are placed in one or more fractures formed in a subterranean zone. The term “on the fly” is used herein to mean that a flowing stream is continuously introduced into another flowing stream so that the streams are combined and mixed while continuing to flow as a single stream, as part of the on-going treatment. Coating the proppant particles with the hardenable resin or tackifying agents, and mixing the coated proppant particles with the fracturing fluid are preferably performed on the fly. Such mixing can also be described as “real-time mixing.” As is well understood by those skilled in the art, such mixing may also be accomplished by batch or partial batch mixing.
Coating compounds are typically mixed with proppant in the blender's sand screws; indeed, a common way to perform mechanical mixing is in a mill. An example of an on-the-fly mixing method is described in U.S. Pat. No. 6,962,200 wherein, a container holds a liquid hardenable resin component, while another container holds a liquid hardening agent component. The liquid materials in the containers are transported to a static mixer. Control of the total and relative amounts of resin component and hardening agent component is achieved through the use of liquid additive pumps and flow meters. In a preferred embodiment, the flow meters are computer-controlled to ensure accurate metering, corresponding to the concentration of proppant and pumping rate of proppant slurry and to allow for a rapid shutdown of on-the-fly mixing when necessary.
A static mixer mixes the resin component and hardening agent component into a single, hardenable resin composition that is then transported through a conveyance means which is partially located inside a proppant hopper. The mixer can be any means known in the art for mixing two liquid streams. The conveyance means can be any means known in the art for conveying particulate material, for example, a proppant screw conveyor. The proppant may be transported to the hopper by any suitable means known in the art. The proppant is removed from a bulk container via conveyor belt, after which it enters the hopper. Inside the conveyance means the coating is mixed with the proppant to form coated proppant particles.
Where conveyance means is a sand screw, the coating is coated onto the proppant by the auger action of the sand screw itself. The coated proppant particles are transported by the sand screw to a blender tub. The coated proppant particles from the hopper may be transported to a blender tub by any means known in the art. In a preferred embodiment, the transport rate of coated proppant particles from sand hopper to blender tub is computer-controlled to ensure accurate metering and to allow for a rapid shutdown of on-the-fly mixing when necessary. Also transported to the blender tub is a fracturing fluid.
While these conventional methods of producing fracturing mixtures containing coated proppant have been successful, vigorous agitation and mixing must be performed in the blender and sand screw to ensure uniform coating of the abrasive proppant with viscous coatings. This results in wear and tear on the mixing equipment.
Problems associated with using sand screws for coating chemicals on proppant: 1) tackiness of coating chemicals tends to cause the sand screw to lock up during fracturing treatment with slow turning of the screw at low proppant concentrations, and 2) tackiness or curing of coating chemicals on sand screw requires its thorough cleaning to prevent it from locking up after each usage. The constant maintenance of sand screw requires time and resources.
Accordingly, there is a need for improved methods and apparatus for making mixtures of fracturing fluid with coated proppant therein.